cmdline/variables.md: the syntax is tweaked

Closes #530

cmdline/variables.md

@@ -35,13 +35,21 @@ You can assign the contents of a plain text file to a variable, too:
 
     curl --variable varName@filename
 
-Starting in curl 8.12.0, you can get a byte range from a given file by
-appending `;[N-M]` to the file name, where `N` and `M` are numerical byte
-offsets into the file where the second number can be omitted to mean until end
-of file. For example, get the contents from a file from byte offset 100 to and
-including byte offset 200:
+Starting in curl 8.12.0, you can get a byte range from content by appending
+`[N-M]` to the variable name, where `N` and `M` are numerical byte offsets
+into the content where the second number can be omitted to mean until end of
+data. For example, get the contents from a file from byte offset 100 to 199,
+inclusive:
 
-    curl --variable "varName@filename;[100-200]"
+    curl --variable "varName[100-199]@filename"
+
+Alternatively, get offset three to twelve from a plain text:
+
+    curl --variable "varName[3-12]=thefulltexttogetrangefrom"
+
+Given a byte range that has no data results in an empty string. Asking for a
+range that is larger than the content makes curl use the piece of the data
+that exists.
 
 ## Expand